My condo building in Boston was built in 1905, and converted from a hotel in the 1950's. Repairing some damaged hardwood in the livingroom the contractor found that the hardwood was nailed directly to the joists, no subfloor of any kind. A surprise!

At the opposite end, a hallway has a sloping floor, side to side, which I would like to correct. I am loathe to try to 'match' the color of the original with new (nailed to sistered, leveled joists), and don't have the height necessary to add subfloor plus new topping (tile or wood) as the front door swings over the sloping floor, one on each side.

The question: I have thought of pretending the hardwood floor is the subfloor, shimming it with purlins every two inches (yes, correct), and screwing concrete board to that and then setting tile. (I think I have height just for that, but not for subfloor, floor, backerboard, then tile.) Do-able?

I'm guessing this is a terrible idea, but it's all I can think of. I can shave the closet door (on the downslope side) but not the other door, which is the front door to the unit, is metal, and can't be easily altered. I'd love to bring up the original hardwood, sister the joists to level, then put the wood back down, but I suspect I will lose a lot in the process and don't want it to look like a patch job.

Suggestions, even brickbats cheerfully accepted. (Note: area is an entrance hallway, about 12'x4', sagging in the middle and level at both ends, and typical of older buildings in the area. 5th floor, can't "jack up" the building.)

4 Answers
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Old houses have 3/4'' thick wood planks as subfloor, not 8x4 sheets of plywood like today's houses, so I would say that it's ok to consider your hardwood floor the subfloor. That's if you're putting another wood floor on top.

However, for the purpose of tiling you need an additional layer of plywood to reduce deflection that will cause the grout to crack eventually. And on top of that you would install your cement board. The cement board itself provides no structural support and is only used as a decoupling between a wood subfloor and the tiles, so the lateral movement of the wood is not going to cause the tiles to pull up. You can always use a durock membrane to reduce thickness, but you still need a floor that will not deflect.

If you really want to do this right you would take out the old floor down to the joist and install some OSB and then do what you want...

My 1926 foursquare has floor like that (well it is not sagging so much), I have red oak hardwood nailed directly to the floor joists (no subfloor).

I agree with md1337, if you are tiling you will need to add more plywood/OSB to make the floor stiffer. Tile needs a very solid floor so it does not deflect (bend) at all because tiles/grout will crack if the floor under it moves like that.

Are you 100% set on tile? Many other floor covering will be much more forgiving of a subfloor that deflects a little and they will probably be thinner so you won't have the height issues you are concerned with. If you are not using tile you could get away with treating the old hardwood as your "subfloor".

You are right the matching the floor will be very hard if you are not refinishing the rest at the same time.

No, not set on 'tile', but if I can't match the wood, tile seems the only option. (Well, except carpet, which would be odd in a place that's otherwise 100% hardwood except kitchen and baths). If I take up the hardwood, I expect to lose at least 25% to breakage. There is a small closet, but probably not enough to supply what I lose. One thought: a tile perimeter, with inset of the original hardwood. Still have the 'tile' issue, I suppose, even though they would be at the walls, where the deflection would be minimal. Thanks for the opinions. More welcome, obviously.
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Eric StarkJun 11 '12 at 17:37

The below are from my limited experience. I got my first home just a couple of years ago, and I haven't worked on wooden framed homes before. You've been warned :)

Shimming the Joists

I would do the below. This trades off extra labor for lower material cost. I've done this and it worked out quite well on a grad student budget :)

Tear everything out to bare joists.

Get a rotating laser level, attach to one of the joists close to the geometric center of the room, to get a reference plane of light. Set it high enough that it clears all the joists. Ensure that this plane of light is level. Use a water level to confirm -- cheap clear plastic tubing filled with water with some food coloring added to it works well. You don't want the shims to be less than 1/8" thick for sure, perhaps even 1/4".

The cheapest thing from a home improvement store will work just fine as long as you won't have direct sunlight coming in. You can return it to the store later :)

Starting from the outermost joists going in, create shims. You do this by clamping down a piece of 2x4 (or wider) lumber on top of the joist. That will be your shim stock. Then mark the laser line on the attached shim stock. Just mark a couple of points, then use a ruler to draw a straight line through them. Cut the shim off, and mark it and the joist with the same number. You must choose a scheme and stick to it or you'll mess it up. Example: the numbers should be at one end of the shim, on the side not facing the joist. Put same number on the joist, right under the number on the shim. Shims get unique numbers. Mark the end of the shim on the joist as well. Leave 1/4" room from the wall baseplate to the shim.

Glue and nail down the shims to the shimmed joists using 6d nails every 12", so as not to split them. Use good construction adhesive.

Attach tongue-and-groove plywood sheeting on top of the joists using screws. You want to drive 3.5" long screws every 6" along the joists. Use plywood rated for floors. I bought the stuff that has 5 stripes sprayed on the edge, don't recall the thickness, it was the thickest tongue-and-groove in the store. Ensure at least 1/8" spacing between sheets of plywood.

Edit: I'd also use a bead of glue between the plywood and the shimmed joist. Cost little, but is a bit of extra insurance against squeaks. Remember: squeaks are where two flexing members rub against each other. Glue and screw, and they can't move anymore, they become one.

You must have a good electric screwdriver with two batteries. One for powering the tool while the other is recharged. Old/weak batteries won't work. Do not use a mains-powered drill, you will likely burn it out. You can use mains powered screwdriver of course. Make sure you get a box of driver bits, unless there's a driver bit supplied in each box of your screws. Calculate how many screws you need -- you'll need many! I find that star- or square-head screws are easier to drive than the ones with Phillips- (cross-) heads. Try it out on a small box of screws. If they constantly destroy your driver bits, you'll need different screws, and perhaps different bit shape. Those screws are supposed to go in easy. If you have to kill yourself pushing down on the driver, something's wrong with the screw/bit combo.

Install Ditra on the plywood following their Installation Handbook. Ensure that your plywood thickness and joist spacing match their recommendations. Nominally you'd use latex mortar (modified thinset) to attach Ditra to plywood, and unmodified thinset to attach tiles to Ditra.

Ditra is hard to beat IMHO. No need for any other layers.

Sistering the Joists

Sistering the joists is IMHO an overkill. The reason is that the tiles only care about deformation along their length, not along the entire length of the floor joist. A tile won't know how far even a 100ft span would sag, all it cares is that it doesn't sag too far along its length. In fact, it doesn't even care about the sag, but about radius of curvature. Admittedly on a pin-supported, uniformly loaded beam like a floor joist the smallest radius (biggest curvature) occurs in the middle, as that's where the largest bending moment happens to be. This would need to be one heck of a long hallway for it to be of concern. That's my opinion so far.

The only way I managed to crack a tile on Ditra was when lifting the entire floor off the foundation wall to replace some rotten lumber. In one place I had to push directly against the floor sheeting, couldn't get to the joist itself. I find that with Ditra the biggest requirement is to have the sheeting (plywood, etc) attached very well to the joists.

Should you decide to sister the joists, it must be done right. See below for ideas.

The sistering lumber can be same width or even wider, it'll only make it stiffer. Say if you have 2x10 joists, you can sister them up with 2x12s extending downwards below the original joists. Just make sure you won't hit your head while in the basement. If there's ceiling underneath, then this obviously won't work, unless you want to lower the ceiling, that is.

Using a rotating laser level is very helpful since you may need to compensate for bowing of sister lumber. It's hard to check otherwise, unless you have a straightedge as long as your hallway. A level is too short.

In absence of a laser level, you can use a thin, well-stretched thread, or thin steel wire. Precisely mark points an inch below the chosen top surface of the sister lumber. Place marks every 24". Then drive thin nails into the points at the ends, and mount your thread/wire. You'll then use it to tweak the lumber to align all of the marked points with the reference line.

Instead of the laser plane hitting the joists directly, move it up an inch or so above the tops of top-be-sisters, and use a ruler or tape measure to make sure the light plane is same height everywhere. Have plenty of clamps -- at least one every 2-3 feet.

You may need to bend the sisters as you go to make them straight. Inspect your lumber when you buy, but even then it may be hard for it to be straight on a long stretch. You want to keep the sisters within 1/32" of where they belong -- at least that's what I'd do. Adjusting it may well be hard, since you're doing it in the direction where the joist is the stiffest. You will need to use your screws to temporarily retain the already drilled part of the sister.

Drill tight fitting screw holes through the sister joist and the original one. A bolt every 12 or 18 inches, 0.5" bolts sound right to me. Use 2-3 C-clamps to hold the joist in place for drilling of the first hole, but afterwards you insert the bolt and can remove all but one of the clamps to drill the second hole. Then probably you can remove the remaining clamp. As you drill the holes and insert the bolts, you'll need to have a helper bend the sister up/down to level the area around where the next screw hole will be.

If you're alone, you'll need a bottle jack and some scrap 2x2 and 2x4s or 2x6s. Make a closed cage of 2x4s to go under and around the joists and the bottle jack. The inside height of the cage is, thus: height of the joists, height of two 2x2s, height of the bottle jack, plus an extra inch.

To lift up, use a loose piece of 2x2 to put between the cage and the bottom of the sister. Place another 2x2 on top of the original joist. The bottle jack goes on top of that. Extend to lift up the sister.

To push down, a loose 2x2 goes between the cage and the bottom of the joist. Another 2x2 goes on top of the sister. Bottle jack on top, extent to push the sister down.

When done with all the screw holes, remove the bolts to unclamp the sister. Put a good wavy bead of construction adhesive on facing sides of both sister and the joist. Use the bolts to reassemble.

Get a flat-end wrench to counterhold the bolts, and a 1/2" drive 6 point socket with a long ratchet to tighten the nuts. Two flat washers on each hole go right against the wood on both ends of the hole. Add a split spring washer between the flat washer and the nut to prevent certain unscrewing due to repetitive flexing of the joists.

If you think about it for a bit, it can definitely be a one-man job, and can even be done in the dead of the night, should you happen to have hard-sleepers in the house (I do and I did).

I love this answer and will upvote anyway but I would make a slight change. If you have the joists exposed anyway I would sister them so they are stronger. Doing all this work to shim the floor but then have it keep saging (or at least still deflect too much for tile) would really suck. I think it depends on the if the floor joists run the length wise down the hall or across it. If there are only 3 joists running the length of this hallway, I would be worried the middle may still crack tile (even with DITRA). I wonder how thick the floor joists are?
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auujayJun 12 '12 at 20:57

I suggest you remove the hardwood, create the level, sistered joists, then replace the hardwood and use new hardwood flooring make up the 25% you broke. Is modern flooring thick enough for this? If not, you can augment its thickness, or mill this small amount (~5 sf) of flooring from scratch yourself.

You can make a wood border, like the one you were suggesting with tile, or maybe the geometry of the room suggests some other shape for the contrasting area. An oversized doormat shape just inside the door?

You could try matching the color of the new wood, but since you will have chosen the new portion's shape and size deliberately, it will still look okay if it doesn't match.

See listing sheet (no contacts; don't want the tenants harassed.) [link] bit.ly/LSmfgC [/link] They leave mid-July. Carpeting is gone. No hallway pic; it's ~4x12 hallway. I don't know about the floor thickness or have exact measurement on the slope, and will not have full workshop there, just hand tools. I live in TN, property is in Boston. Awkward. I won't know what to do 'til I get there, but want to think it through now. I might do as you suggest: rim the area with a different wood. Dunno. Tile would be my preference for this entry hallway, but nobody here seems to think that works.
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Eric StarkJun 11 '12 at 20:56